I have a Dell Latitude d620 running Windows XPSP2 and have had an intermittent problem with my Intel 3945abg wireless card acquiring an IP address from ANY wireless access point. The connection would be successful, but I could not pick up an IP address and then I'd get the "limited or no connectivity" message. It worked for the first two years, and then stopped abruptly one day. After changing EVERY parameter I could find and reloading the newest drivers over and over, I have found a solution.

I rolled the driver back and my wireless connection immediately acquired an IP address from our Linksys router. I then connected successfully to the hotel network next door as well.

So, my problem was having the wrong, new driver for this card. The one that works is:

12.4.3.9 - 05/28/2009

I hope this helps some other people as I was extremely frustrated for a long time and I have been through lots of discussion forums relating to this very issue.